When exploring for or extracting subterranean resources such as oil, gas, or geothermal energy, and in similar endeavors, it is common to form boreholes in the earth. To form such a borehole 111, an embodiment of which is shown in FIG. 1, a specialized drill bit 112 may be suspended from a derrick 113 by a drill string 114. This drill string 114 may be formed from a plurality of drill pipe sections 115 fastened together end-to-end. As the drill bit 112 is rotated, either at the derrick 113 or by a downhole motor, it may engage and degrade a subterranean formation 116 to form the borehole 111 therethrough. Drilling fluid may be passed along the drill string 114, through each of the drill pipe sections 115, and expelled at the drill bit 112 to cool and lubricate the drill bit 112 as well as carry loose debris to a surface of the borehole 111 through an annulus surrounding the drill string 114.
Various electronic devices, such as sensors, receivers, communicators or other tools, may be disposed along a drill string or at a drill bit. To power such devices, it is known to generate electrical power downhole by converting energy from flowing drilling fluid by means of a generator. One example of such a downhole generator is described in U.S. Pat. No. 8,957,538 to Inman et al. as comprising a turbine located on the axis of a drill pipe, which has outwardly projecting rotor vanes, mounted on a mud-lubricated bearing system to extract energy from the flow. The turbine transmits its mechanical energy via a central shaft to an on-axis electrical generator which houses magnets and coils.
One limitation of this on-axis arrangement, as identified by Inman, is the difficultly of passing devices through the drill string past the generator. Passing devices through the drill string may be desirable when performing surveys, maintenance or fishing operations. To address this problem, Inman provides a detachable section that can be retrieved from the downhole drilling environment to leave an axially-located through bore without removing the entire drill string.
It may be typical in downhole applications employing a turbine similar to the one shown by Inman to pass around 800 gallons/minute (3.028 m3/min) of drilling fluid there past. As the drilling fluid rotates the turbine, it may experience a pressure drop of approximately 5 pounds/square inch (34.47 kPa). Requiring such a large amount of drilling fluid to rotate a downhole turbine may limit a drilling operator's ability to control other drilling operations that may also require a certain amount of drilling fluid.
A need therefore exists for a means of generating electrical energy downhole that requires less fluid flow to operate. An additional need exists for an electrical energy generating device that does not require retrieving a detachable section in order to pass devices through a drill string.